The present invention pertains in general to ion-selective membranes and in particular to insulative layers associated with ion-selective membranes and conductive patterns suitable for fabrication of ion-selective electrodes.
When placed in contact with a solution, ion-selective electrodes provide an electrical output which is a function of the concentration of a particular ion in the solution. In such electrodes an output potential ("Y") is measured between a "sensing element," responsive to the concentration of the particular ion, and a "reference element," held at a constant potential, Y may be plotted against the base 10 logarithm of the concentration of the ion ("X") as a straight line having a slope ("M") and y-axis intercept ("B") as expressed in the Nernst equation: EQU Y=M (log.sub.10 X)+B
Ion-selective electrodes conventionally have an internal reference element of Ag/AgCl immersed in a solution or gel of chloride ion. The chloride ion solution or gel holds the reference element at a constant potential, providing that the chloride concentration and thermodynamic functions, such as temperature and pressure, are held constant. An ion-selective glass or membrane sensing element is placed in contact with the solution or gel to form an interface between the test solution and this internal filling solution. However, this conventional design is complex to manufacture and difficult to miniaturize.
In the fabrication of ion-selective electrodes a major problem is leakage at the interface of the ion-selective membrane and the insulative surface. This leakage causes corrosion and drift of the ion-selective electrode. Various attempts to prevent leakage at the membrane interface are described in the literature. U.S. Pat. No. 4,180,771 describes placing the gate lead on the opposite face of FET device to isolate the ion-sensing area. U.S. Pat. No. 4,449,011 to Kratachvil describes placing an insulating tape around the ion-sensing areas in an attempt to prevent moisture leakage.
U.S. Pat. No. 4,393,130 to Ho et al, provides a dry film photoresist laminate which requires photo processing and etching to form a window around the ion-sensing areas for placement of the ion-selective membrane. A process for encapsulating ion-selective electrodes with thixotropic material having a window for an ion-selective membrane is also disclosed.
U.S. Pat. Nos. 4,456,522 and 4,486,292 to Blackburn describe a method for spinning a polyimide layer onto a conductor which is then chemically etched to leave a floating polyimide mesh. The mesh provides a physical support for a polymeric ion-selective membrane.
U.S. Pat. No. 4,454,007 to Pace describes a system whereby the ion-selective membrane is anchored to a conductor by intersolubilization. However, moisture may penetrate between the membrane and insulating layers to corrode the contacts.
In all these cases effective anchoring of the ion-selective membrane and insulation of the conductors are not achieved. Therefore, new systems for anchoring ion-selective membranes are desirable. The present invention describes a system comprising a plurality of insulative layers deposited over the substrate where at least a portion of one of the layers is intersolubilized with ion-selective membrane. This system is effective for preventing leakage between the membrane and the insulating layers.